- What is ozone?
Ozone is a highly reactive, unstable connection of three oxygen atoms. The word itself is derived from the Greek ozein, wheach means 'to smell'. No without reason, because the gas has a typical odour that is sometimes noticeable in badly ventilated places with many (old) copying machines or after a thunderstorm with a lot of lightning. The chemical formula is O3.
Ozone in the ozone layer (in the stratosphere at a height of 15 to 45 km) protects the earth's surface against the harmful solar UV radiation.
Ozone in the lower air layer (the troposphere) at ground level is formed on hot days by the influence of sunlight (UV of ultraviolet light) on air polluted with nitrous oxides ad volatile organic compounds.
- On which days is there too much ozone?
A number of conditions have to be met before too much ozone occurs in the lower atmosphere (at ground level):
- weather has to be sunny (lots of UV light). Clouds stop UV radiation emitted by the sun ( to a high degree).
- it has to be sufficiently warm (>25 °C).
- wind has to come from continental direction (E,SE,S) with low wind speed.
- enough nitrous oxides (NOx) anc volatile organic compounds (VOC) have to be present in the atmosphere, and in correct proportions.
The (simplified) chemical reaction of ozone formation can be found in the answer of another question. It is important to state that a single day with sunny and warm weather does not lead to exceedances of the ozone treshold most of the time. Longer periods with such conditions are (most often) necessary.
An excess of ozone ('ozone smog') at ground level only occurs in Belgium during the months of May, June, July and August, or exceptionally during the end of April or the first half of September in periods with a high pressure system located above continental Europe combined with eastern to southeastern wind currents towards Western Europe.
An overview of the number of 'ozone days' (i.e. days on which at least one exceedance of the EU treshold value (180 µg/m³) for informing the population was measured in Belgium) can be found here.
Under special meteorological conditions an intrusion of ozone can happen from the free troposphere or even the stratosphere towards the lower atmosphere. This can exeptionally (last occurred in Belgium on the 5th, 6th and 7th of May 1995) cause exceedances of the treshold value. This may also be called 'spring ozone'. Also, powerful storms in spring or autumn, with great downwards air currents can lead to increased ozone concentrations at ground level.
- I thought there was a hole in the ozone layer and thus too little ozone
The hole in the ozone layer is the yearly re-occurring decrease of the thickness of the ozone layer above the South Pole in October-November (the Antarctic spring). As already postulated in the answer onf the first question, an ozone layer exists in the stratosphere, at great height. This 'good' ozone protects us against the harmful solar UV radiation. Without this ozone life would be impossible as a matter of fact. The 'bad' ozone in the lower atmosphere that we thus can inhale, can have detrimental effect (see a following question). The 'good' ozone in the ozone layer is chemically identical to the 'bad' ozone at ground level, yet the chemical reactions leading to formation of ozone in the ozone layer are different from those in the lower atmosphere.
Certain chemical compounds, such as CFC's (Chloro-Fluorà Carbohydrates), bromine compounds, ... can disturb the reactions involved in ozone formation in the ozone layer, which will lead to a decreased concentration of ozone in the ozone layer (or in other words the ozone layer will grow thinner). Because of geographical and climatological reasons, this decrease is most spectacular above the South Pole.
Ozone-depleting substances were frequently used in the chemical industry, for example as coolant or as disinfectant in agriculture, and are very stable chemical compounds, allowing them to remain in the atmosphere for a long time. It can take up to 50 years before they reach the stratosphere and are eventually degraded; Most of these ozone-depleting compounds are forbidden by now, but because of there long lifespan it can still take many years before the ozone layer will regain its original thickness.
The ozone data published on this website are data about the lower atmosphere ('bad' ozone). Data on the stratospheric ('good') ozone cn be found on the KMI website.